How to Choose a Cutting Oil

Neil Hasson, PetroLiance

I'm often asked to recommend cutting oils for metalworking facilities. In most cases, there have been recent performance issues with the existing oil or something has changed in the manufacturing process that precludes its use. New machines, additional contracts, changing environmental concerns, product cancellations, sustainability goals and other factors can all combine to limit the effectiveness or applicability of the current oil.

In order to select the best oil, you need to gather some basic information relevant to the selection criteria. For purposes of simplicity, you need to know the metals in use, the predominant machining operations, basic machine types, tooling specifics, plant processes and chemical restrictions for your facility.

Solicit input from all departments (quality, production, maintenance, safety, packaging, disposal, purchasing, etc.) to identify all critical requirements in advance. Seek the assistance of an experienced, qualified metalworking fluids specialist to identify options and guide you through the selection process.


Some metals are more difficult to machine than others. Stainless steel, exotic alloys and very hard metals demand a very high level of performance from the cutting oil. Other metals, like brass and aluminum, are easy to machine with general-purpose oils.

Where tough, low-machinability metals are involved, you will need highly additized cutting oil with excellent extreme-pressure (EP) and anti-weld capability. Most often, these oils contain active sulfur and chlorine to protect the tooling and ensure good parts finish.

For brass, aluminum, many carbon steels and low-alloy steels, a cutting oil with lubricity additives, friction modifiers and mild EP/anti-weld performance is sufficient. These oils are generally formulated with sulfurized fat (inactive) and/or chlorinated paraffin. Active cutting oils (containing active sulfur) should not be used for brass and aluminum, as they will stain or tarnish the finished parts. Oils formulated for brass and aluminum are often called "non-staining" oils.

Machining Operations

Easy machining operations (turning, forming, drilling, milling, etc.) can be performed at higher speeds and require high levels of cooling with only modest EP capability. The milder operations can be performed with lower viscosity, lightly additized fluids.

Difficult machining operations must be run at lower speeds and require a great deal of anti-weld protection. Oils designed specifically for the most difficult operations, like thread-cutting or broaching, are generally higher in viscosity and loaded with EP additives like active sulfur and chlorine.

Basic Machine Types

The type of machinery will also dictate some of the cutting oil characteristics. For example, screw machines experience heavy cross-contamination between the lube oil and cutting oil. For this reason, these machines frequently run on dual-purpose or tri-purpose oils that can be used in the lube boxes, hydraulics and cutting oil sumps.

Grinders, gun drills and deep-hole drilling machines require lighter viscosity oils for high rates of cooling, good chip and swarf flushing, through-the-tool delivery and high-pressure application without foaming. CNC OEMs may place restrictions on the cutting oil due to potential incompatibility between the cutting fluid and machine components, such as seals. Centerless grinders may require a tougher fluid than surface grinders.

Always be ready to discuss your plant equipment when requesting recommendations for metalworking fluids from your supplier.

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Tooling Specifics

Where specialty tools, coatings or grinding abrasives are in use, review these carefully with your supplier. Some cutting oil additives will not work effectively with particular coatings, and the wrong grinding oil can contribute to wheel loading and other issues. If you are spending the extra money for carbide tooling, cobalt coatings or cubic boron nitride (CBN) grinding wheels, it only makes sense to select a cutting oil that will maximize performance and financial return.

Plant Processes

Cutting oils must meet your production requirements without causing issues in downstream activities. Fluids that are ideal for the machining applications may cause serious issues with cleaning, painting, packaging or disposal. A review of downstream processes, including storage and transportation, will identify key requirements that the metalworking fluid must meet. Identifying these needs early in the selection process will avoid wasting time and money in testing and installing the wrong fluid.

Chemical Restrictions

It is very frustrating to select and test a fluid only to find out later it cannot be used in the facility. Every plant has specific chemical restrictions based upon wastewater disposal, corporate environmental policies, employee health and safety, end-user requirements, and other key concerns. It is critical to identify these limitations in advance of selecting a metalworking fluid.

Even this brief survey of selection criteria demonstrates the thoroughness required to choose the proper cutting oil. The best approach is the team approach, with key personnel from the manufacturing facility and fluid supplier involved. Oftentimes, critical demands compete with each other; satisfying one violates another. For this reason, selecting any process fluid is a balancing act, requiring technical acumen and sound business logic. The process demands more than a phone call and a last-minute decision.

Cutting oil affects your equipment, people, environment and business reputation. If you take the time to choose carefully, purchase quality products from reputable suppliers and seek out competent technical advice, the rewards will be considerable. Getting it right will improve your product quality, lower manufacturing costs and avoid costly downstream issues. 

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